import { Meta } from '@storybook/blocks';

import * as NestedScrollViewStories from '../stories/NestedScrollView.stories';

<Meta of={NestedScrollViewStories} />

# NestedScrollView

A scrolling view inside of which can be nested other scrolling views, with their scroll positions being intrinsically linked.

## Example

```dart liveslice=NestedScrollView mockup=iPhone7
List<String> _tabs = ['One', 'Two', 'Three'];

DefaultTabController(
  length: _tabs.length, // This is the number of tabs.
  child: NestedScrollView(
    headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
      // These are the slivers that show up in the "outer" scroll view.
      return <Widget>[
        SliverOverlapAbsorber(
          // This widget takes the overlapping behavior of the SliverAppBar,
          // and redirects it to the SliverOverlapInjector below. If it is
          // missing, then it is possible for the nested "inner" scroll view
          // below to end up under the SliverAppBar even when the inner
          // scroll view thinks it has not been scrolled.
          // This is not necessary if the "headerSliverBuilder" only builds
          // widgets that do not overlap the next sliver.
          handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
          sliver: SliverAppBar(
            title: const Text('Books'), // This is the title in the app bar.
            pinned: true,
            expandedHeight: 150.0,
            // The "forceElevated" property causes the SliverAppBar to show
            // a shadow. The "innerBoxIsScrolled" parameter is true when the
            // inner scroll view is scrolled beyond its "zero" point, i.e.
            // when it appears to be scrolled below the SliverAppBar.
            // Without this, there are cases where the shadow would appear
            // or not appear inappropriately, because the SliverAppBar is
            // not actually aware of the precise position of the inner
            // scroll views.
            forceElevated: innerBoxIsScrolled,
            bottom: TabBar(
              // These are the widgets to put in each tab in the tab bar.
              tabs: _tabs.map((String name) => Tab(text: name)).toList(),
            ),
          ),
        ),
      ];
    },
    body: TabBarView(
      // These are the contents of the tab views, below the tabs.
      children: _tabs.map((String name) {
        return SafeArea(
          top: false,
          bottom: false,
          child: Builder(
            // This Builder is needed to provide a BuildContext that is "inside"
            // the NestedScrollView, so that sliverOverlapAbsorberHandleFor() can
            // find the NestedScrollView.
            builder: (BuildContext context) {
              return CustomScrollView(
                // The "controller" and "primary" members should be left
                // unset, so that the NestedScrollView can control this
                // inner scroll view.
                // If the "controller" property is set, then this scroll
                // view will not be associated with the NestedScrollView.
                // The PageStorageKey should be unique to this ScrollView;
                // it allows the list to remember its scroll position when
                // the tab view is not on the screen.
                key: PageStorageKey<String>(name),
                slivers: <Widget>[
                  SliverOverlapInjector(
                    // This is the flip side of the SliverOverlapAbsorber above.
                    handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
                  ),
                  SliverPadding(
                    padding: const EdgeInsets.all(8.0),
                    // In this example, the inner scroll view has
                    // fixed-height list items, hence the use of
                    // SliverFixedExtentList. However, one could use any
                    // sliver widget here, e.g. SliverList or SliverGrid.
                    sliver: SliverFixedExtentList(
                      // The items in this example are fixed to 48 pixels
                      // high. This matches the Material Design spec for
                      // ListTile widgets.
                      itemExtent: 48.0,
                      delegate: SliverChildBuilderDelegate(
                        (BuildContext context, int index) {
                          // This builder is called for each child.
                          // In this example, we just number each list item.
                          return ListTile(
                            title: Text('Item $index'),
                          );
                        },
                        // The childCount of the SliverChildBuilderDelegate
                        // specifies how many children this inner list
                        // has. In this example, each tab has a list of
                        // exactly 30 items, but this is arbitrary.
                        childCount: 30,
                      ),
                    ),
                  ),
                ],
              );
            },
          ),
        );
      }).toList(),
    ),
  ),
)
```

## Related Links

- https://api.flutter.dev/flutter/widgets/NestedScrollView-class.html
